The manufacture of many industrially made products involves the use of water, resulting in the generation of an aqueous waste stream as a by-product of the manufacturing process. In order to limit environmental challenges associated with disposal of waste streams, organic pollutants are often removed from waste streams by aerobic biological water treatment i.e. by cultivating microbial organisms to convert the pollutants present in aqueous waste streams to produce microbial cell mass, carbon dioxide and other metabolites, and water that is relatively free of contaminants.
A typical biological wastewater treatment process involves the cultivation of microbial cells within an aerated reactor comprising the aqueous wastewater and microbial cells grown in suspension in the aqueous wastewater. The microbial cells produced in the aeration reactor are normally allowed to overflow continuously into a solid-liquid separator (e.g. a gravity clarifier, dissolved air flotation vessel, or membrane-based system) generating a clear effluent and microbial biomass, frequently referred to as “activated sludge”. The effluent is then discharged in a local waterway, injected underground or discharged in any other appropriate manner, and the microbial mass, is in part returned to the aeration reactor (frequently referred to as “return activated sludge” or “RAS”), and in part disposed as solid waste (frequently referred to as “waste activated sludge” or “WAS”).
Known wastewater treatment processes exhibit several drawbacks. First, the waste activated sludge component must be disposed and the disposal costs are frequently a significant cost component in the operation of a wastewater plant. However, waste activated sludge may be converted to valuable products e.g. animal feed products (see: U.S. Pat. No. 7,931,806), thus significantly improving the wastewater treatment economics.
Second, wastewater streams exhibit a substantial degree of variation in constituent composition. Diurnal variations, seasonal variations, and variations caused by variability in the upstream manufacturing processes, for example, all have the potential to impact the concentration of organic compounds, inorganic micronutrients, and other wastewater constituents. Depending on the constituent composition of the wastewater stream, a wastewater stream may be more or less suitable as an efficient medium to cultivate microbial cells in aeration basins.
Third, it is frequently observed that filamentous microbial organisms grow in conventional aerated biological wastewater systems, a phenomenon known as filamentous bulking (see: US Patent Application 2011/0139714). The growth of filamentous microbial organisms results in poor separation and poor compaction of the microbial mass during solid-liquid separation in the clarification process, in turn potentially resulting in undesirable carry-over of microbial mass into the effluent, as well as an undesirable loss of waste activated sludge and thus loss of raw material for conversion to valuable products.
Fourth, conventional wastewater treatment practices commonly focus on reducing the total biomass produced to reduce disposal costs. This is often achieved by keeping the cells in the aerobic wastewater treatment system for longer periods of time in order to mineralize them into carbon dioxide in the aeration basins. This ages the cells and results, on average, in lower intracellular protein levels. It is also commonly observed that this method of holding the cells for a long period can result in poorer removal rates of nutrients, especially nitrogen and phosphorus, the concentration of which in many instances must be controlled in the effluent before release in order to meet environmental standards. Furthermore, this approach requires large amounts of oxygen to mineralize the organic compounds, which adds significant cost to wastewater treatment. Reducing the mean cell age (also known as “mean cell residence time” or “MCRT”) results in an increased mass of cells that are also younger which, when grown appropriately, have the potential to contain high concentrations of protein and other nutritional components.
Fifth, conventional wastewater treatment methods require the use of substantial amounts of oxygen, which is typically supplied by the operation of an aeration blower; however this adds capital and operational costs.
Additionally, some food-processing-derived wastewaters contain compounds that negatively impact the growth of microorganisms. These compounds are most often derived from the plant materials being processed into the food products and result in poor wastewater treatment and lower so called “mixed liquor suspended solids” (or “MLSS”) concentrations versus what would normally be observed.
Thus, there are still significant shortcomings in the conventional methodologies for the production of microbial mass in wastewater treatment operations, limiting the total amounts of recoverable value-added products, and sometimes resulting in effluent contaminated with undesirably high concentrations of nutrients and/or microbial organisms.